Brain scanDavid Deutsch, father of quantum computing

“I OCCASIONALLY go down and look at the experiments being done in the basement of the Clarendon Lab, and it’s incredible.” David Deutsch, of the University of Oxford, is the sort of theoretical physicist who comes up with ideas that shock and confound his experimentalist colleagues—and then seems rather endearingly shocked and confounded by what they are doing. “Last year I saw their ion-trap experiment, where they were experimenting on a single calcium atom,” he says. “The idea of not just accessing but manipulating it, in incredibly subtle ways, is something I totally assumed would never happen. Now they do it routinely.”

Such trapped ions are candidates for the innards of eventual powerful quantum computers. These will be the crowning glory of the quantum theory of computation, a field founded on a 1985 paper by Dr Deutsch. He thinks the widely predicted “quantum supremacy” that eventually puts a quantum computation incontrovertibly ahead of a classical one will be momentous for scientists and laymen alike. He brushes off the fervent debate about whether the commercially available D-Wave computer offers a speed advantage. “If it works, it works in a completely different way that cannot be expressed classically. This is a fundamentally new way of harnessing nature. To me, it’s secondary how fast it is.”

Still, these are steps towards a powerful, universal quantum computer that could solve a lot of thorny problems. To describe such a device properly is to account not only for the states of each of its constituent bits but also for all the couplings between them, for each is entangled with every other. A good-sized one would maintain and manipulate a number of these states that is greater than the number of atoms in the known universe. For that reason, Dr Deutsch has long maintained that a quantum computer would serve as proof positive of universes beyond the known: the “many-worlds interpretation”. This controversial hypothesis suggests that every time an event can have multiple quantum outcomes, all of them occur, each “made real” in its own, separate world.

At the same time, quantum computation, and the quantum-mechanical theory from which it springs, are all subsumed in a newer idea that Dr Deutsch is pursuing. He contends that what he calls his “constructor theory” provides a perspective that will lead to the rewriting of physics altogether. As with classical computer science, quantum computation and even genetics, it is based on the role of information. But rather than letting physical laws define what is and is not possible, as science does now, constructor theory asserts that those laws actually arise from what is and is not possible.

From observed possibilities, a mathematical object called a constructor can be fashioned. Operating with and on these constructors gives rise to what Dr Deutsch reckons is a theory even more fundamental than quantum mechanics. He is enthusiastic about the theory’s potential to upend the very foundations of science, but concedes that testing it experimentally remains a distant possibility. Then again, a few decades ago he would have said the same thing about quantum computers.

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